This invention relates to an at-wavelength EUVL mask blank inspection system based on scanning of a focused EUV beam onto the mask blank surface and correlating the intensity of the specular reflection and defect scattering to potential defects at and below the surface of the mask blank.
Extreme ultraviolet lithography (EUVL) is a promising technology for integrated circuit fabrication for feature sizes less than 0.1 xcexcm. It is an optical projection lithography scheme using short wavelength radiation with all-reflective optics based on multilayer coatings. An EUVL reticle is also reflective, consisting of a multilayer coated substrate and a patterned absorber layer. In order to insure the integrity of the printed pattern, the reticle has to be free of any critical defect which can occur either on the absorber pattern or on the mask blank itself. While several techniques have been proposed for correcting defects in the absorber overlayer pattern, there is no known repair technology for defects in the multilayer coating. Moreover, the defects in the multilayer coating have been shown to be more detrimental than absorber defects of the same dimension and those defects may be very hard to detect. A recent defect printability study by Lin and Bokor, J. Vac.Soc. and Tech. B 15(6) pp2467 Nov/Dec 1997, showed that a totally opaque defect in the mask blank as small as 40 nm will produce 10% process window reduction for a 0.1 xcexcm contact hole. The ability to produce and certify mask blanks with low defect density is a critical issue for the economic viability of EUVL technology.
The present invention is based in part on the recognition that when a focused EUV beam is incident on a defective region of a mask blank, three possible phenomena can occur. The defect will induce an intensity reduction in the specularly reflected beam, scatter incoming photons into an off-specular direction, and change the amplitude and phase of the electric field at the surface which can be monitored through the change in the photoemission current. The magnitude of these changes will depend on the incident beam size, and the nature, extent and size of the defect. Inspection of the mask blank is performed by scanning the mask blank with EUV light focused to a small spot (typically a few microns or less in diameter), while measuring the reflected beam intensity (bright field detection), the scattered beam intensity (dark-field detection) and/or the change in the photoemission current. Defects in the mask will cause changes in the electric field on the surface of the substrate. This can be detected by monitoring changes in the photoemission current.
Accordingly, in one aspect the invention is directed to a method for detecting defects at or below the surface of a mask substrate that includes the steps of:
directing extreme ultraviolet (EUV) radiation, typically having a wavelength of 5-15 nm, on a region of the surface of the mask substrate;
measuring the intensity of the specular reflection and the intensity of the defect scattering from the region; and
determining whether defects are present on the mask substrate surface.
In a preferred embodiment, the size and phase of the defects is determined by developing functional relationships between the measured intensities of the specular reflection and the defect scattering and the size of the defect on the region and the phase information for the region.
The feasibility of the at-wavelength mask blank inspection system has been demonstrated. From initial scans on a programmed phase defect mask, a prototype system is shown to be sensitive to phase defects down to 0.5 by 0.8 xcexcm with a beam size 4xc3x976 xcexcm. Currently, improvements to the beam spot size are being actively pursued to achieve an EUV beam spot with about 1 xcexcm diameter.